Fig.
1: view of the fan and part of the catchment of l’Illgraben (Picture
E. Bardou).

L’Illgraben
is an inhabited steep valley on the left bank of the Rhône valley suffering
high erosion rate (Fig. 1 and 2). The Illgraben catchment area is constituted
of strongly fractured quartzites, evaporities and carbonates. This fracturing
is caused by the Rhône-Simplon fault or by one of its digitations still
active today.